This invention relates to pumping systems and more particularly to pumping systems used as diluters or as dispensers.
In one class of pumping system used as a diluter or as a dispenser, pulses are generated as the pump motor rotates so that a number of electrical pulses are generated representing the number of rotations of the motor. The volume to be dispensed is represented by a number of pulses and compared with the pulses generated by the motor. The motor is stopped after a predetermined number of rotations or portions of rotations so that the programmed volume is dispensed or aspirated.
In this type of prior art dispenser or diluter, the pump is a peristaltic pump and the motor drives a rotor which compresses and releases tubing for the pumping action. The speed of the rotary motor is proportional to the flow rate and volume dispensed or aspirated in a fixed period of time.
The prior art dispensers or diluters utilizing peristaltic pumps have a disadvantage in that they are not precisely repeatable from dose to dose. The lack of repeatability is caused partly by the lack of reliability of the tubing used in peristaltic pumps since the amount of flexing of the tubing with the rollers driven by the motor may vary from time to time as the walls of the tubing are worked and changed.
Piston pumps are known to have high repeatability. However, the piston pumps: (1) have return strokes during which no pumping action occurs in a single chamber pump; (2) require a transmission mechanism to convert rotary to linear motion when the primary source of power is a rotary motor; and (3) may require complicated valving, particularly with multiple chamber pumps. The transmission and valves are sources of non-linearity.
Moreover, when low volumes of liquids are to be dispensed or aspirated by a piston pump, if the piston is changing from a retraction to an extension or vice versa, the dosage is affected greatly. If a very low volume is to be dispensed and the stroke is in the wrong direction, nothing may be dispensed, or if a larger volume is to be dispensed, the amount is difficult to control without an exact knowledge of the portion of the stroke of the piston.
It is known to control the speed of pumps by causing a disk to rotate with the pump motor and counting the pulses for comparison with a standard. For example, U.S. Pat. No. 3,985,021 to Achener et al, granted Oct. 12, 1976, discloses a piston pump to be used for high performance chromatography utilizing such a disk. The speed is controlled by means of the pulses which are unequally spaced on the disk to speed up the return cycle and reduce pulsations of liquid by so doing.
This type of prior art pump has the disadvantage of creating volume error when used as a dispenser and, while it controls the rate of pumping relatively well for chromatography, it does not accurately control the amount of dosage because emphasis is placed on a continuous stream of fluid at a continuous rate of flow rather than on a controlled volume. Thus, the indicia on the disk do not control the length of a stroke but instead the speed of movement of the piston so that indicia are present when, in fact, there is only a return stroke. In the return or chamber filling stroke, the indicia are fewer in number than during a pumping stroke to increase speed during the return stroke but some are sensed.